In recent years, there have been proposed various information processing systems following the advancement of information-oriented society and there have been proposed information recording devices for use in those information processing systems. In such information recording devices, there have been required larger information recording capacities and higher information recording densities for the purpose of miniaturization and higher performances of the information processing systems.
In magnetic recording devices, as such information recording devices, represented by HDDs (hard disk drives) using magnetic recording media as recording media, the information recording capacity exceeding 30 GB (gigabytes) has been required per 2.5-inch magnetic disk.
In order to improve the information recording capacity of a magnetic recording medium, it is necessary to improve the performances of both the magnetic recording medium and a magnetic head that records/reproduces an information signal with respect to this magnetic recording medium. In order to satisfy such a requirement with a magnetic recording medium, it is necessary to realize an areal information recording density exceeding 60 gigabits per inch2 (60 Gbits/inch2).
Incidentally, in magnetic recording devices currently widely used, use is made of a magnetic recording medium comprising a magnetic recording layer of the so-called in-plane magnetic recording type (longitudinal magnetic recording type or horizontal magnetic recording type). In this in-plane magnetic recording type, the magnetization direction in the magnetic recording layer becomes a direction approximately parallel to the main surface portion of the magnetic recording medium.
In the in-plane magnetic recording type, there is a possibility that even if attempting to perform information recording at high areal recording density such as 60 gigabits per inch2 by decreasing the size of crystal grains in a magnetic recording layer, the influence of demagnetization fields between the adjacent crystal grains increases so that satisfactory recording cannot be carried out. Further, since it is necessary to reduce the thickness of a magnetic recording layer for decreasing the size of crystal grains in the magnetic recording layer, there is a problem that thermal fluctuation failure tends to occur due to thermal magnetic aftereffect. If the thermal fluctuation failure becomes significant, recording magnetization is attenuated with the lapse of time and, finally, recorded information cannot be normally reproduced.
In view of this, in recent years, it has been proposed to employ the perpendicular magnetic recording type, instead of the in-plane magnetic recording type, in magnetic recording media. In the perpendicular magnetic recording type, even if the areal recording density is increased, the resistance to thermal fluctuation failure is high. Therefore, the perpendicular magnetic recording type is a desirable recording/reproducing type for achieving information recording at high areal recording density.
In order to employ the perpendicular magnetic recording type instead of the in-plane magnetic recording type in magnetic recording media, it is necessary to largely change the structure of a magnetic recording layer. That is, in a perpendicular magnetic recording medium employing the perpendicular magnetic recording type, it is necessary to orient an easy magnetization axis of a magnetic recording layer being a hard magnetic layer in a perpendicular direction (normal direction) with respect to the main surface of the magnetic recording medium. For example, in the case of forming a magnetic recording layer using a cobalt (Co)-based ferromagnetic material, the easy magnetization axis of the magnetic recording layer becomes the c-axis in the hexagonal closest packed (hcp) crystal structure of cobalt. Therefore, in this case, it is necessary to orient the c-axis of the cobalt crystal structure in a perpendicular direction with respect to the main surface of the magnetic recording medium.
There has been proposed, as such a perpendicular magnetic recording medium, a so-called two-layer type perpendicular magnetic recording medium having a soft magnetic layer made of a soft magnetic substance or ferromagnetic microcrystal on a nonmagnetic substrate and further having a magnetic recording layer made of a hard magnetic substance on the soft magnetic layer. This soft magnetic layer serves to conduct magnetic flux emitted from a magnetic head and perpendicularly transmitted through the magnetic recording layer or magnetic flux perpendicularly emitted from the magnetic recording layer, to a magnetic path leading to the magnetic head. That is, in such a two-layer type perpendicular magnetic recording medium, a suitable magnetic circuit can be formed among the magnetic head, the magnetic recording layer, and the soft magnetic layer at the time of magnetic recording, so that it is possible to obtain an action wherein the soft magnetic layer assists the magnetic recording based on the mirror image effect. Therefore, providing the soft magnetic layer between the nonmagnetic substrate and the magnetic recording layer is considered to be a desirable structure as a perpendicular magnetic recording medium.
In the meantime, a reduction in noise has conventionally been an aim in perpendicular magnetic recording media and is essential also in the perpendicular magnetic recording medium having the soft magnetic layer between the nonmagnetic substrate and the magnetic recording layer. This noise is generated from both the magnetic recording layer and the soft magnetic layer and, particularly, spike-like noise (spike noise) generated from the soft magnetic layer and medium noise have been a problem.
In view of this, proposals have conventionally been made for reducing such noise. For example, Patent Document 1 describes a perpendicular magnetic recording medium having a backing magnetic layer between a nonmagnetic substrate and a magnetic recording layer, wherein the backing magnetic layer comprises a pair of ferromagnetic films having the same thickness and laminated through a nonmagnetic layer therebetween.
In this perpendicular magnetic recording medium, the pair of ferromagnetic films of the backing magnetic layer are antiparallel-coupled to each other and, according to Patent Document 1, it is described that leakage magnetic flux generated from magnetic domain walls in the backing magnetic layer is prevented from entering a magnetic head and the magnetic domain walls in the backing magnetic layer are fixed so as not to move easily, so that medium noise caused by the backing magnetic layer is reduced.
Patent Document 2 describes a perpendicular magnetic recording medium having a soft magnetic underlayer between a nonmagnetic substrate and a magnetic recording layer. This soft magnetic underlayer comprises a first soft magnetic layer, a magnetic domain control layer including at least an antiferromagnetic layer, and a second soft magnetic layer. In this perpendicular magnetic recording medium, the ratio (d1/d2) between a thickness d1 of the first soft magnetic layer and a thickness d2 of the second soft magnetic layer is set to or more and or less.
In this perpendicular magnetic recording medium, in the case of being formed as a perpendicular magnetic recording disk, if a magnetic field is applied in a radial direction of the nonmagnetic substrate, since the soft magnetic underlayer has the magnetic domain control layer including the antiferromagnetic layer, a magnetization curve of the soft magnetic underlayer shifts in the magnetic field direction. Since a coercive force Hc of the soft magnetic underlayer derived from this magnetization curve is smaller than an exchange bias magnetic field (shift amount) Hex, the magnetization does not take a single value in a zero magnetic field, that is, the hysteresis of the magnetization curve does not cross the zero magnetic field.
According to Patent Document 2, it is described that because of the fact that the hysteresis of the magnetization curve does not cross the zero magnetic field, a uniaxial magnetic anisotropy with its easy magnetization axis in the radial direction of the nonmagnetic substrate and a unidirectional magnetic anisotropy with its easy magnetization direction being the direction of the magnetic field are produced in the soft magnetic underlayer, so that magnetic domain walls in the soft magnetic underlayer are driven to an end portion side of the nonmagnetic substrate to provide a pseudo-single-domain state, thereby suppressing generation of spike noise in a data area.
Further, Non-Patent Document 1 describes a perpendicular magnetic recording medium having a pair of soft magnetic layers, laminated through a nonmagnetic layer therebetween, between a nonmagnetic substrate and a magnetic recording layer. In this perpendicular magnetic recording medium, the pair of soft magnetic layers are antiparallel-coupled to each other. According to Non-Patent Document 1, it is described that spike noise is suppressed when the thicknesses of the soft magnetic layers are equal to each other, while, spike noise is generated when a difference in thickness between the soft magnetic layers increases.    Patent Document 1: Japanese Unexamined Patent Application Publication (JP-A) No. 2001-331920    Patent Document 2: Japanese Unexamined Patent Application Publication (JP-A) No. 2004-348849    Non-Patent Document 1: The 28th Magnetics Society of Japan Scientific Conference Summaries (2004) pp. 612 to 613